A battery pack according to the present invention includes a battery, a control unit, a power supply circuit, and a supply source selection unit. The power supply circuit is configured to receive electric power selectively from one of the battery and a charger, and generate a voltage for driving the control unit. The supply source selection unit is configured to select the charger as a power supply source to the power supply circuit when the battery is in a state of being able to supply electric power to the power supply circuit, the battery pack is attached to the charger and it is possible to supply electric power to the power supply circuit also from the charger.